The present invention relates to filtration media which may be membranous and nonmembranous. The membranous media may be macroporous or microporous. Microporous membranes are usually defined as thin walled structures having typically spongy morphologies with a narrow pore size distribution. The mean pore sizes for microporous membranes are between 0.01 .mu.m and 10 .mu.m. Microporous membranes have found widespread use in removing fine particulate matter such as dust and bacteria from liquids and gases.
The nonmembranous porous materials are materials such as woven and nonwoven fabrics, glass fiber mats, melt blown mats, felts, and the like. These porous nonmembranous materials are presently being used extensively as filtration media or as prefilters for membranous filtrations.
The function of most conventional filtration media is based on physical sieving wherein particles are captured by membrane pores smaller in diameter than the size of the particle or by impinging on small cells defined by fibers in the coarser filtration media. Although this principle is adequate for many applications, the need for removing finer and finer particles is increasing. This is especially true in the high-technology industries such as the electronics and pharmaceutical industries.
One method of removing smaller particles is simply to use membranes with correspondingly smaller pore sizes. This method is limited because the smaller pore size filtration media inherently possess a substantially reduced flow rate with concomitant increased expenditure in membrane area and operating costs.
An alternative which obviates the increased costs of filtration media having smaller and smaller pore sizes is the concept of combining the sieving filtration mechanism with an active absorption mechanism. High flow rates can be maintained using this combined concept while particle capture is much more efficient than indicated by the rated pore size of the filter.
One of the possible mechanisms for such absorption is electrokinetic capture. This is achieved by imparting an appropriate zeta potential to the filter's internal and external surfaces.
When a charged surface is immersed in an aqueous medium or other polar medium, a charge double layer will form at the solid-liquid interface. One component of the double layer is the charged solid surface. The other component or layer is a counter ionic region in the liquid. When the solid and liquid are set in relative motion, a potential difference will develop between the mobile and immobile regions in the liquid close to the surface. This potential, called the zeta potential is given for example for a fluid flowing through a charged porous medium by: ##EQU1## where .zeta. is the zeta potential, .eta. is solution viscosity, D is dielectric constant, E is the streaming potential, P is the driving pressure and K is the specific conductance of the solution.
The zeta potential can be positive or negative depending on the charge of the surface. Most naturally occurring particle suspensions have negative zeta potentials. Therefore, such particles will be attracted to and absorbed by surfaces with positive zeta potentials. Applying a positive zeta potential to the available surface (both external and internal) of filter media will greatly improve capture capacity for small colloidal particles.
The U.S. Pat. No. 4,473,474 to Ostreicher et al issued Sep. 25, 1984, is an example of a charge modified microporous membrane and process for charge modifying the membrane. The European patent application 83300618.4 to Pall Corporation, published Oct. 5, 1983 discloses a modified skinless hydrophobic microporous polyamide membrane. Both the Ostreicher et al and Pall Corporation disclosures use the same principle of adding a preformed commercial quaternary ammonium polymer to a membrane. The U.S. Pat. No. 4,473,475 to Barnes Jr. et al, issued Sep. 25, 1984 also discloses a cationic charged modified microporous membrane and method of making the same. The Barnes, Jr. et al patent utilizes known epoxy chemistry to post-treat a membrane.
Each of these membranes and methods of making the same suffer from a number of drawbacks including slow flush-out times to obtain a high resistivity permeate, low charge capacity, and pH sensitivity of capture ability.
Some of the aforementioned patents disclose methods and membranes which are pH dependent in order to maintain a charged surface. The aforementioned membranes depend on the basicity of an amine to acquire a charge in aqueous media. This is exemplified by the formula: ##STR1## These systems may lose their charge at high pH because of the equilibrium illustrated above. Hence, these systems do not maintain a permanent charged surface.
Poly(vinylpyridine) and high molecular weight polyalkylene-imine polymers have not been used before in making or modifying microporous membranes. Very little work has been done on the use of these polymers in membranes in general. Some work has been done on using a true polymer blend of poly(vinylpyridine) and cellulose acetate to make reverse osmosis membranes. These membranes are designed to separate salt from an aqueous solution and are entirely different from those discussed regarding the present invention. The main effect seen was an improvement in membrane water throughput compared to a membrane without poly(vinylpyridine). (Aptel et al, J-Appl. Pol. Sci., 25, 1969 (1980)).